1. Field of the Invention
This invention relates to a catalyst for the removal of nitrogen oxides and to a method for the removal of nitrogen oxides by the use of the catalyst. More particularly, it relates to a honeycomb catalyst to be used for the removal of nitrogen oxides contained in waste gases discharged from boilers, heating furnaces, gas turbines, diesel engines, and various industrial processes and to a method for efficient removal of such nitrogen oxides by the use of the catalyst.
2. Description of the Prior Art
In the methods currently available for the removal of nitrogen oxides from waste gases, the method of selective catalytic reduction which attains selective removal of nitrogen oxides even from a waste gas of a high oxygen content, operates effectively with only a small amount of a reducing agent, and allows use of ammonia as a reducing agent by reason of economy has been predominating.
As respects the shape of the catalyst to be used in this method of selective catalytic reduction, the catalyst in the honeycomb construction proves to be effective for the reason that the dust in the waste gas is not readily deposited on the catalyst bed and the pressure loss suffered by the catalyst bed is small. At present, therefore, the honeycomb catalyst is in extensive utility. Concerning the method for effective removal of nitrogen oxides by the use of a honeycomb catalyst, it is disclosed in Japanese Patent Publication SHO 54(1979)-29, 419, for example, that when the through holes in the honeycomb catalyst are so defined as to possess an equivalent diameter in the range of 2 to 30 mm and an opening ratio in the range of 50 to 80% and produces a gas flow rate in the range of 0.5 to 60 m/sec, the honeycomb catalyst experiences no clogging of the through holes with the dust, suffers only from a small pressure loss, and enjoys a high ratio of removal of nitrogen oxides (hereinafter referred to as "nitrogen removal ratio").
In recent years, the environmental pollution with nitrogen oxides represented by the acidic rain has been worsening on the global scale. In the circumstances, an earnest desire has been expressed to perfect as a measure for decreasing the emanation of nitrogen oxides a technique for nitrogen removal waste gases with high efficiency at a low cost. For the sake of this technique, it is extremely important to promote compaction of a nitrogen removal apparatus by decreasing the amount of catalyst required therefor and, at the same time, lower the pressure loss of the catalyst bed to the fullest possible extent and decrease the power consumption required for the operation of a fan.
The method disclosed in Japanese Patent Publication SHO 54(1979)-29, 419 is not enough at all to fulfil the requirement described above. The desirability of developing a catalyst for the removal of nitrogen oxides which possesses a still higher capacity for denitrification and suffers only from a small pressure loss or a method for efficient removal of nitrogen oxides has been finding enthusiastic recognition.
Incidentally, it is widely known that in proportion as the opening ratio in the cross section of a given honeycomb catalyst is increased, the thickness of partition walls separating the through holes is inevitably decreased (hereinafter referred to occasionally as "cell wall thickness") and, as the result, the pressure loss of the catalyst bed is lowered and the geometric surface area of the catalyst is increased, with the result that the nitrogen removal activity is enhanced. For the pressure loss of the catalyst bed to be suppressed to the fullest possible extent, it is necessary that the opening ratio of the catalyst should be increased as much as possible. The present inventors' study has revealed that when the opening ratio of a given catalyst exceeds 80% and, as an inevitable consequence, the cell wall thickness is decreased, the catalyst's nitrogen removal activity is liable to decrease conversely.
An object of the present invention, therefore, is to provide a novel catalyst for the removal of nitrogen oxides and a method for the removal of nitrogen oxides by the use of the catalyst.
Another object of this invention is to provide a honeycomb catalyst to be used for the removal of nitrogen oxides contained in waste gas and a method for efficient removal of the nitrogen oxides by the use of this catalyst.
Yet another object of this invention is to provide a catalyst for the removal of nitrogen oxides which allows ample suppression of pressure loss, excels in the ability to nitrify waste gas, the enjoys highly satisfactory durability and a method for efficient removal of nitrogen oxides by the use of this catalyst.